In the conversion of coal to synthetic fuels by direct liquefaction, the coal is mixed with a recycle solvent and is hydrogenated in a three phase reactor at temperatures in the range 750.degree. to 880.degree. F. and pressures in the range 1000 to 3000 psi. The process is generally known as SRC-I, solvent refined coal having the acronym SRC. In this and similar processes, coal is mixed with solvent at low temperature (typically from 150.degree. to 450.degree. F.) and atmosphere pressure. The resulting slurry is pumped to a high pressure (for example, 2500 psi) and is then preheated in heat exchangers to a temperature of approximately 500.degree. F. This temperature is chosen to be sufficiently low that dissolution and reaction of the coal has not commenced.
Hydrogen gas is then added to form a three phase mixture which is heated in a fired heater, prior to entry to the reactor vessel. This fired heater is a critical component in the direct liquefaction of coal. Because of the high operating pressure and temperature and the erosive corrosive nature of the coal slurry, expensive materials are required for the fired heater tubes making this unit a major cost item in the liquefaction process.
Further, reaction of the coal system commences in the fired heater, and coke formation from the coal products may occur at any location where very high temperatures are encountered, for example, at the surface of the heated tube wall. The avoidance of coke formation is an important consideration in the design since coke buildup will eventually cause tube plugging and may, in an extreme case, lead to tube wall temperatures sufficiently high to allow rupture of the tube.
A primary objective of this present invention is to optimize the tube wall temperature throughout the heater such that coking is minimized while utilizing the least possible surface in order to minimize the cost.
Problems which must be addressed in the design of a fired heater for coal slurry service are:
1. Swelling and dissolution of coal in solvent as it passes through the temperature zone from 500.degree. to 650.degree. F. lead to a large peak in the magnitude of the coal slurry viscosity. This region of high viscosity, termed the "gel region", has a low heat transfer coefficient. Immediately following this inlet and intermediate temperature region in the heater, the heat transfer coefficient increases rapidly. This variation of heat transfer must be accomodated in the design of the heater while avoiding the possibility of coke formation. PA1 2. The flow of a slurry is preferably handled in a horizontal tube configuration. This prevents the possibility of flow blockage by settling which may occur in vertical tubes. PA1 3. Tube erosion by the slurry must be avoided by limiting flow velocities and avoiding short radius tube bends. PA1 1. Less surface is required; PA1 2. Lower and more uniform maximum film temperatures are possible; PA1 3. A 2.times.50% duty arrangement can be utilized with a minimum increase in cost.
A design of a fired heater which recognizies the latter two problems has been described in U.S. Pat. No. 4,013,402. In this patent, there is disclosed a radiant heater having a tube configuration containing the slurry flow in a horizontal racetrack arrangement with long radius return bends. This arrangement is not entirely satisfactory since it may cause unacceptable high tube wall temperatures in the inlet temperature range from 500.degree. to 650.degree. F. The heater configuration generates a maximum heat flux at the inlet area of the slurry tube circuit where the inside heat transfer coefficient is low. This could lead to high slurry film temperatures and coking in the coal gel formation zone of the heater.
Thus, this prior art design does not allow for variations of tube side heat transfer coefficients. Substantial variations in such heat transfer coefficients are probable in the heating of a coal slurry of the indicated type due to the effect of viscosity changes of the slurry during solvent absorption by the coal and the dissolution process. Hence, if the heat flux in the heater is maintained low to avoid the likelihood of coke formation, then the upper zones of the heater may require an unnecessarily large surface area.
As an alternative to the radiant fired heater, convective designs are known which have the advantage of obtaining more uniform heat transfer to the tubes and are not subject to the occurrence of local hot zones in the furnace due to such problems as flame impingement. In such a convective heater, hot discharge gases from a burner or burners are mixed with recirculated cooler gases. The gas mixture is passed over the outside of the tube bank in which the coal slurry is heated. The cooled exit gases are then divided into two streams, one part of the gas is exhausted to atmosphere, the second part provides the recirculated gases to mix with the burner discharge gases. In the tube bank, several circuits are arranged in parallel with the pipes transverse to the flue gas flow. The coal slurry flowing in the pipes has a flow configuration either cocurrent with or countercurrent to the flow gases.
In this type of prior heater, in order to obtain uniform flow of the flue gases, it is usually necessary to have several pipe circuits in parallel. This, in combination with other economic considerations which require minimization of the number of pump and pipe coal slurry flow circuits, determines that such heaters are primarily suitable for very large duties--for example, only one heater may be required for a 6000 T/Day coal liquefaction plant. This may be considered a disadvantage since the plant onstream capability may be adversely affected by an individual circuit failure.
The co- or countercurrent flow arrangement is most efficient when the heat transfer coefficient for the process fluid only exhibits small variations throughout the heater. Then, by the use of varying tube spacing and added external surface (fins) it is possible to optimize the heat flux distribution. When the process fluid heat transfer varies widely, as for a coal slurry, such an optimization is not practical and the heater must generally be designed for the lowest prevailing heat flux.
The heater construction in accordance with the invention is designed to obviate the above-discussed problems of the prior art heaters. To this end, heat flux variations on the fired side are minimized by utilizing a convective design. In addition, the fired side temperature profile is selected to minimize the possibility of the slurry film temperature exceeding the coking temperature. Moreover, the relative heat inputs to zones of the furnace are controllable. Furthermore, the design in accordance with the invention permits the use of two 50% duty units without a large cost premium when compared with a single 100% unit.
Briefly stated, the convective heater in accordance with the invention is constructed so that the flue gas flow is divided into two parallel paths across the slurry tube circuits. Three parallel slurry tube circuits are used and are arranged so that each tube circuit enters the heater at a central point in one path of the flue gas circuit and flows co-current to the flue gas exit. The tube circuit then crosses to the other flue gas path and flows counter-current to a location near its inlet. The tube circuit then returns to the first flue gas path and flows co-current to leave the heater adjacent to the entry location.
The advantages of the mixed flow arrangement of the invention are as follows:
In accordance with another feature of the invention there is provided a special cross-over arrangement of the return bends for the tube circuits to provide a compact tube arrangement while retaining a long radius for the return bends. This also provides a more uniform temperature relationship between the three tube circuits by interchange of the heat transfer contact between the coal slurry and different zones of the flue gas as the process flows progress through the heater.
Prior art heater constructions acknowledged herein are those disclosed in U.S. Pat. Nos. 1,833,130; 2,514,084; 2,669,099; 2,955,807; 3,258,204; 3,623,549; 4,201,191; and 4,230,177. These patents do not disclose the heater design of the present invention wherein the heating gas flow of a convective heater is divided into two flow passes with the tube circuit comprising a mixed co-current, counter-current arrangement.